RENTON — No team in the league has forced opponents into more third-and-long situations than Seattle.

That fact is evidence of the Seahawks' fortitude against the run and an ability to hold their ground on first and second downs.

No team in the league has allowed opponents to convert more third-and-long plays than the Seahawks. That fact points to an underlying weakness of the defense, a persistent inability to get off the field that could endanger Seattle's playoff hopes.

"It's disturbing," coach Pete Carroll said, "that we're not able to be like we are in the rest of our game. We'll try to take the turn here. We've had some deep discussions about it, and see if we can get it fixed right away."

The Seahawks allowed the Detroit Lions to convert 12 of the 16 third downs they faced Sunday, the highest percentage by any Seattle opponent since December 2004.

Those are the kind of numbers you'd expect when an opponent repeatedly faces the equivalent of gimme putts on third down. That wasn't the case, as six of Detroit's 12 third-down conversions came on plays in which the Lions needed seven yards or more.

It was a continuation of a seasonlong trend. Third-and-long is a situation when Seattle tends to take it in the shorts.

NFL teams convert an average of 26.1 percent when it's third-and-seven or longer this season, before Monday's game, according to STATS LLC. The Seahawks have given up a first down 39.3 percent of the time, which is a much bigger difference that it sounds.

If Seattle maintained the NFL average, it would have allowed opponents to convert 16 third downs that required seven or more yards this season. The Seahawks have allowed 24.

That's a difference of eight plays. Eight drives that were extended. Eight times a stop would have forced a kick and instead resulted in a first down. In a season in which Seattle's four losses have come by a combined total of 21 points, those eight plays could have made all the difference.

Besides, Seattle is not an average defense. The Seahawks have allowed the third-fewest points per game in the NFL and the fifth-fewest yards. They went two consecutive road games without allowing an offensive touchdown, yet even before Sunday's game, the Seahawks were a decidedly mediocre No. 16 in third-down defense.

Now, they're No. 27, the only one of the league's top-six scoring defenses to rank outside the top 10 in that category.

Diagnosing the problem isn't as important as fixing it, though. The problem is that there hasn't been one consistent theme to the third-down struggles.

"It would be easy if we got beat in man coverage the whole time," Carroll said, "or it was the zone stuff, or the pressures."

That last category is key because defensive difficulties on third down are often attributed to an inability to pressure the opposing passer.

"I don't disagree with that thought," Carroll said.

The Seahawks had two sacks in Detroit, one by defensive tackle Brandon Mebane and the other by linebacker Leroy Hill. The Seahawks missed defensive tackle Jason Jones — a staple of their nickel pass rush — who was out with an ankle injury. They also mistimed several blitzes, according to Carroll.

It's not like Seattle's personnel undergoes a wholesale overhaul on third down. The same guys who are the reason Seattle has allowed the third-fewest points in the league are largely the same guys on the field on third down.

But after that performance Sunday, the pressure is on Seattle to improve the third-down defense that threatens its season.